With an increasing degree of integration and increasing performance of today's semiconductor devices, higher quality is required for silicon wafers (hereinafter simply referred to as “wafers”) used as substrates for semiconductor devices.
Specifically, it is required that a denuded zone (hereinafter referred to as the “DZ layer”), which forms the surface layer of the wafer in which semiconductor devices are formed, is completely free of oxygen precipitates which are compounds of silicon and oxygen that dissolves into the silicon crystal during growth of the silicon crystal from a crucible for storing silicon melt, and also completely free of void defects which are aggregates of vacancies introduced into the crystal during growth of the crystal. This is because oxygen precipitates could act as leak current sources and deteriorate the electrical properties of the semiconductor devices, while void defects could form dents in the surface of the wafer which could result in the breakage of wiring formed on the surface.
On the other hand, it is required that in the bulk layer, disposed deeper than the DZ layer, oxygen precipitates exist at a density higher than a predetermined level. This is because, while the oxygen precipitates in the DZ layer deteriorate, as explained above, the electrical properties of the semiconductor devices, the oxygen precipitates in the bulk layer serve as gettering sources to remove heavy metals that adhere to the surface of the wafer, thus improving the electrical properties of the device, and also increase the mechanical strength of the wafer by preventing dislocation that could cause plastic deformation during heat treatment of the wafer.
The density distribution of the oxygen precipitates in the wafer depth direction largely depends heavily on the distribution, in the wafer depth direction, of point defects (especially vacancies) that form during a high-temperature rapid thermal process (hereinafter abbreviated to “RTP”) performed on wafers. In JP Patent Publication 2009-16864A, a wafer sliced from a crystal grown by the Czochralski process is subjected to RTP in an argon or hydrogen atmosphere (see paragraph 0037 of JP Patent Publication 2009-16864A). The wafer formed by such RTP is free of oxygen precipitates in the DZ layer, while oxygen precipitates exist in sufficient density in the bulk region of this wafer (see FIGS. 7A-8 of JP Patent Publication 2009-16864A).